Neurodegenerative diseases are disorders characterized by gradual and progressive loss of nerve cells. Examples of these diseases include Alzheimer's disease, Parkinson's disease, and Huntington's disease. Among them, Huntington's disease (HD) is an autosomal dominant disorder caused by a CAG trinucleotide expansion in exon 1 of the Huntingtin (Htt) gene. See, e.g., Perutz et al., Trends Biochem. Sci. 1999;24:58–63; and Rubinsztein et al., J. Med. Genet. 1999;36:265–270. HD patients have abnormal body movement, dementia, and psychiatric problems. Medications, such as dopamine blockers, reduce the abnormal movement and behavior, but do not stop the regression. Thus, there is a need for a new drug that effectively treats HD and other neurodegenerative diseases.